Homo ergaster

These people were generally tall and slender and may also have been relatively hairless. Not everyone accepts this species name, some still prefer to use the term African Homo erectus.

Background to discovery

Age

The core group within this species lived between 1.5 and 1.9 million years ago although some classifications include additional individuals that extends their range to between about 700,000 and 2 million years ago.

Important fossil discoveries

Homo ergaster was first proposed as a new species in 1975 after scientists re-examined a fossil jaw previously identified as Homo habilis. Colin Groves and Vratislav Mazák noticed some unique features about this jaw that made it different to our other human ancestors. These same features were later recognised in a group of fossils that had initially been thought to be early forms of Homo erectus from Africa. All these fossils have now been reclassified as Homo ergaster. New fossil discoveries have since been made and this species is now represented by fossils from males and females as well as adults and juveniles.

Important specimens

‘Turkana Boy’ KNM-WT 15000 – skeleton discovered in 1984 by Kamoya Kimeu in Nariokotome, West Turkana, Kenya. The Turkana Boy or ‘Nariokotome Boy’ as he is sometimes called, lived about 1.5 million years ago. He was about 8 to 10 years of age when he died but was already 1.6 metres tall and may have reached 1.85 metres as an adult. Almost 90% of his skeleton was recovered and has provided valuable information on this species’ body size, proportions and development. The Turkana Boy had a tall, slender body adapted for striding out across the extensive savannah plains. He also had a more human-like face with a nose that projected outwards and a larger braincase.

SK 847 – a partial skull discovered in 1969 in Swartkrans, South Africa by Ronald Clark. This skull was found in a cave with many fossils from another species, Paranthropus robustus. Stone tools and burned bones were also found at this site. The tool maker was probably Homo ergaster. Fire may have been used here about 1.5 million years ago by Homo ergaster, although the burned bones may have resulted from a natural fire rather than from a controlled man-made fire.

KNM-ER 3733 – skull discovered in 1975 by Bernard Ngeneo and Richard Leakey in Koobi Fora, East Turkana, Kenya. This is the skull of an adult female. Females had less robust features compared with males such as ‘Turkana Boy’.

KNM-ER 992 – a lower jaw discovered in 1971 by Bernard Ngeneo in Koobi Fora, East Turkana, Kenya. This lower jaw is the ‘type specimen’ or official representative of this species. It was first classified as Homo habilis, but was reclassified as Homo ergaster in 1975 because it showed advanced features such as a lightly built jaw and relatively small premolar and molar teeth.

KNM-ER 42700 - A 1.5-million-year-old skull of a young adult discovered in Ileret in Kenya in 2000 (described in 2007). The skull has a very small brain of about 691cc, the smallest for any Homo ergaster. This indicates that this species came in a variety of sizes, with males being much larger than females, which was unexpected for this species. It also shows features that had previously only been found in Asian Homo erectus, such as the ridge on the frontal and parietal skull bones. This mix of traits blurs the distinction between Asian Homo erectus and African Homo ergaster and has caused some experts to rethink whether these should be separate species.

BSN49/P27 – a female pelvis from Gona, Afar in Ethiopia, dated to 1.8 million years old. The size of this pelvis suggest the female was quite short at only about 130cm in height, much smaller than has been estimated for females prior to this discovery. The size and shape also indicate the female could have given birth to a young with a brain 30-50% the size of an adult’s. This suggests that the growth rate of the brain in the womb was similar to that of a modern human but slowed down in the first few years of life to a rate intermediate between modern humans and living chimpanzees.

Various fossils found in Eurasia at Dmanisi in the Republic of Georgia may belong to Homo ergaster. These Dmanisi fossils are significant because they currently represent the earliest evidence for the emergence of early humans from Africa into Eurasia 1.75 million years ago. Key specimens include: Skull D2700 (discovered in 2001) with a brain size of 600 cc; Skull D2280 (discovered in 1999) with a brain szie of 780 cc and features similar to Homo ergaster specimens KNM-WT 15000 and KNM-ER 3733; and Skull D2282 (discovered in 1999) with a brain size of about 650 cc and features similar to KNM-WT 15000 and KNM-ER 3733.

Fossil footprints from Ileret, Kenya, dated to 1.5 million years ago. These are the oldest known evidence of an essentially modern human-like foot anatomy and differ from the Laetoli footprints left by australopithecines 3.6 million years ago. The size and shape suggest that they were made by Homo ergaster, which also makes them the oldest surviving footprints made by a human species.

What the name means

Homo, is a Latin word meaning ‘human’ or ‘man’. It is the same genus or group name as the one given to modern humans, which indicates the close relationship between this species and our own.

The word ergaster is based on a Greek word meaning ‘work’, so the name Homo ergaster means ‘workman’. This name was used because large stone tools were found near some of its fossils.

Distribution

Fossils of this species have been found in Africa and Eurasia. Important sites include regions around Lake Turkana and Lake Victoria, Koobi Fora, Nariokotome, Olorgesailie, Swartkrans and Dmanisi, Georgia.

Relationships with other species

Some people do not recognise Homo ergaster as a species and instead classify these fossils as Homo erectus. Those who do accept Homo ergaster consider this species to be the common ancestor of two groups of humans that took different evolutionary paths. One of these groups was Homo erectus, the other group ultimately became our own species Homo sapiens.

Some fossils including the ‘type specimen’ (a jaw known as KNM-ER 992) were formerly classified as Homo habilis.

Finds from Dmanisi in Georgia are currently attributed by most scientists to this species, although new finds led to the suggestion in 2002 that these belong in a new species, Homo georgicus. However, this is not widely accepted.

More recently, skull KNM-ER 42700, dating to 1.5 million years old and discovered in Ileret in Kenya in 2000 (but described in 2007), blurs the distinction between Asian Homo erectus and African Homo ergaster. It shows features that had previously only been found in Asian Homo erectus specimens, such as the ridge on the frontal and parietal skull bones. This mix of traits caused some experts to rethink the whether these should be separate species.

Key physical features

This species’ tall, long-legged body, with a flatter face, a projecting nose and a somewhat expanded brain was well along the evolutionary path leading to modern humans but it still possessed a number of intermediate features.

Body size and shape

the body is usually considered to be tall and slender with long legs which may have been an adaptation to maximise cooling of the body in a hot, dry environment. However, a pelvis found in 2000 suggests that females at least were broad-hipped and short.

females grew to about 160 centimetres in height whereas males reached about 180 centimetres in height.

the body may have been relatively hairless as a way of improving body cooling by sweating.

ribcage was like that of modern humans in being barrel-shaped rather than cone-shaped as in earlier species. Along with changes to the shoulders, chest and waist, this new body shape improved the body’s balance and made it possible to run.

Brain

average brain size was approximately 860 cubic centimetres and made up about 1.6% of their body weight

Skull

had developed a more human-like shape including a higher, more domed cranium or braincase. Unlike modern humans, the cranium had a moderate post-orbital constriction (indents behind the eye sockets). This feature is linked to brain size. As our ancestors’ brains expanded, their skulls became fuller and more rounded with increasingly smaller post-orbital constrictions.

face projected outward but to a smaller degree than in earlier ancestors

distinct double-arched brow ridge lay above the eyes and a relatively distinct groove was located between the brow ridge and forehead

nose was human-like for the first time. It now projected outward whereas earlier species had flat noses

Jaws and teeth

jaw was shorter and more lightly built than those of earlier species, resulting in a flatter, shorter face

like earlier species, the front of the lower jaw sloped backward and did not form a pointed chin like that of modern humans

arrangement of the teeth within the jaws was intermediate between that of apes and modern humans in that the side rows of teeth were much further apart at the back of the jaw than at the front

canine teeth were modern in form, being short and blunt like those of modern humans

premolar and molar teeth were smaller and more human-like than those of earlier species.

Backbone and pelvis

pelvis was shaped like that of a modern human and was relatively narrow compared with species such as Australopithecus afarensis. This probably allowed more efficient movement on two legs.

a female pelvis specimen indicates that at least some females had quite broad-hipped bodies rather than the tall narrow body suggested by the Turkana Boy skeleton. This pelvis shares some features with A.afarensis.

body was vertically aligned above the pelvis.

vertebrae from the neck region of the backbone show that the spinal cord was thinner than in modern humans. This may indicate that this species had limited speech abilities due to the lack of nerves needed for the complex control of breathing while speaking.

Limbs

unlike earlier species, the legs were much longer than the arms, so the limb proportions were similar to those of modern humans

tree climbing adaptations of earlier species had been lost and had given way to a long-legged striding walk that was an efficient way to move about and made it easier to travel longer distances

an ability to run on two legs is suggested by a variety of limb features as well as changes to the shoulders, chest and waist that enabled the body to stay balanced during prolonged running

Lifestyle

Culture and technology

The technology of Homo ergaster became more advanced with the production of new kinds of stone tools. Other aspects of their behaviour also showed some significant changes, including the possible use of fire and increased levels of physical activity.

Tools

Large stone tools including hand axes, cleavers and picks (classified as Mode 2 technology) were manufactured. To make these tools, large stone flakes were produced and these were then shaped on two sides to produce sharp edges. This improved technology created more durable tools that maintained their sharpness longer than earlier types of tools. Microscopic examination has shown their tools were mainly used on meat, bone, animal hides and wood.

Mode 2 technology includes straight-edged cleavers, pointed picks and hand axes. These tools are often called Acheulean stone tools after St Acheul in France where similar tools were first discovered during the 1800s. These tools were suitable for heavy duty work including processing bones for marrow, butchering large mammals and woodworking. This new technology was developed by Homo ergaster in Africa and was an improvement on the very simple stone choppers (Mode 1 technology) that earlier ancestors such as Homo habilis had been using for about one million years. Later, Homo heidelbergensis continued to use this technology in Africa and they also took this technology with them when they spread into Eurasia. One of the richest Acheulean stone tool sites in Africa is Olorgesailie, Kenya. Dating shows these tools were made over 700,000 years ago and they may even be up to 900,000 years old.

Fire

Fire may have been used as long as 1.5 million years ago for cooking and warmth but whether this was a controlled use of fire is not certain. Charcoal, burnt earth, and charred bones found associated with Homo ergaster fossils may have resulted from naturally occurring fires rather than from intentionally lit and controlled fires.

Recent reports (Current Anthropology vol 52, 4, August 2011) of discoveries in Wonderwerk Cave, South Africa, suggest controlled use of fire may have been occuring by 1.7 million years ago. Stratified deposits contain burnt stones, charred-calcined bones and traces of ash that indicate repeated burning events. The discoverers conclude that the fire-makers, most likely Homo ergaster, regularly gathered around the fire to prepare and cook food and also for social reasons.

Social behaviour

None of the Homo ergaster skeletons that have been found so far were deliberately buried. There is evidence however, that they did care for living members of their group who were sick or injured, but they did not seem to be concerned with their welfare after death.

It is probable that these people lived in social groups based on family bonds. A comparison with groups of primates living today suggests that these humans were moving away from a dominant-male social structure. Their developmental rates show that they took longer to mature to adulthood than modern apes, but not as long as modern humans. This feature suggests that Homo ergaster had an extended childhood period in which to complete development to maturity.

Environment and diet

About 1.8 million years ago, the climate over most of Africa became drier and more seasonal with extensive savannahs. Homo ergaster was the first human species to take advantage of these more arid and open environments.

This species’ narrower pelvis and rib cage suggests that they had a smaller gut than earlier species such as Australopithecus afarensis. The development of a smaller gut and a bigger brain required more nourishing food and this suggests that they may have included more meat in their diets.

In the dry savannah environment, plant tubers would probably have been an important part of the diet. These tough vegetables may have been processed using their improved technology as their smaller molar teeth imply that they ate foods that required less chewing.